Command Formations can be taken as normal Formations, as well as included in a Gladius Strike Force. The first option is Strike Force Command, which just allows you to take one named characters, Captain, or Chaplain, accompanied by Honor Guard, a Command Squad, and an Emperor's Champion as you wish. This doesn't give you any special bonuses. It just gives you some flexibility in HQ choices in your Gladius Strike Force.
The Reclusiam Command Squad is Chaplain and Command squad that must be mounted in a Razorback. The Chaplain must join the Command squad and can't leave it during the course of the battle. The squad gains Crusader, which is always a nice bonus for a combat unit, and the Chaplain gains Stirring Rhetoric. This grants re-rolled failed to hit rolls to all Space Marine units within 6" of the Chaplain, so essentially, he gas a Hatred aura. This seems like a pretty good benefit for assault armies, but the weakness of this Formation seems to be the Razorback. It's not a good transport for an assault unit, and is easy for the enemy to neutralize it. If your enemy can kill the Razorback and keep the Reclusiam Command squad from supporting your assault units, then you aren't getting any benefit from it. You're better off just adding a Chaplain directly to your assault unit.
The Librarius Conclave is the Formation that everyone is talking about. It consists of 3 to 5 Librarians, who gain Empyric Channelling. This allows you to pick one Librarian each turn, who may cast any powers known by members of the Conclave within 12" and gains +1 to rolls to harness Warp Charge for each member of the Conclave within 12". The downside is that any member of the Conclave within 12" of the chosen member can't cast powers that turn (there's some argument about whether they can cast at the end of the phase, but I think it's pretty clear that they aren't supposed to be able to cast). This doesn't really matter because their powers can be cast by the nominated Librarian with a big bonus to their casting rolls. Just pick the Librarian in the best position and he can use up all of your Warp Charge dice. I've often lamented that Space Marines didn't have the capability to compete in the psychic phase. The Librarius Conclave gives them that option. They won't generate as many Warp Charge as Eldar or Daemons, but they'll use them more efficiently.